Preheat oven to 400°F (200°C). Wrap whole beets in foil and bake until tender, about 45 minutes.
Allow beets to cool, then peel and slice into rounds or strips.
Heat oil in a large skillet over medium heat. Add minced garlic and sliced red onion, sauté for 2-3 minutes.
Add chopped kale to the skillet and cook until soft and wilted, about 5 minutes.
Warm tortillas in a dry skillet or microwave until pliable.
Assemble tacos by layering beet slices, sautéed kale mixture, avocado slices, jalapeño, and cilantro in tortillas.
Serve immediately with lime wedges or salsa if desired.